The present invention relates to optical measuring devices, and more specifically to an optical gauge for measuring the level of a material with great accuracy.
Optical gauges are employed in many applications where mechnical contact is undesirable or impossible. For example, in processing molten glass or in the continuous casting of metals, it is necessary to maintain the surface of the material between desired limits as the process is carried out. Optical gauges, wherein only a light beam contacts the process material, are the natural choice for such applications. Lasers provide a desirable light source for optical gauges of this type since they emit a concentrated beam of coherent radiation at a wave length selected for best performance in the particular environment in which the gauge is to be employed.
In the measurement of the level of materials such as molten glass, it is necessary to maintain a high degree of accuracy in an atmosphere of combustion gases, smoke, water vapor, dust and the like. The use of a laser is advantageous in such a situation, allowing measurements at distances of many feet. The use of a laser system in such measurements is described in U.S. Pat. No. 4,461,676 to Charles King which issued on July 24, 1984, the disclosure of which is hereby incorporated herein by reference. The optical system described in that patent includes a laser light source, a stationary parabolic mirror, a rotating plane mirror, a photodetector and an electronics package for establishing a time period in response to actuation of the photodetector and for converting the time period to the desired measurement. Between the laser and the photodetector, the beam may strike the parabolic and the plane mirrors and the object or material being measured in any desired order, but in any event the axis of rotation of the plane mirror intersects the optical axis of the parabolic mirror at the latter's focal point.
The rotating mirror is carried on the output shaft of a synchronous motor which also carries an opaque disc having a slot or cutout area therein. A light source and a photodetector are fixedly positioned on opposite sides of the disc so that the cutout portion of the disc passes therebetween during a fixed portion of each revolution of the motor. When the device is used to measure the surface level of a material such as molten glass, the laser source is positioned to project a beam at an angle upon the surface of the glass for reflection to the parabolic mirror. As the surface level varies, the reflected beam will strike the parabolic mirror at different points along a radial path. The beam is reflected from the parabolic mirror to a point on the rotational axis of the rotating plane mirror, and is again reflected by the plane mirror along a moving path which intersects a second photodetector during a portion of its travel. As the position of the beam on the parabolic mirror varies with variations in the surface level of the glass, the angular position of the plane mirror at the time the beam intersects the second photodetector will vary. Thus, the time between activation of the first detector, which serves as an index detector, and the activation of the second photodetector varies, providing a direct indication of the distance between the actual surface level and a previously assigned reference surface level. In this configuration, the laser is mounted on one side of the material being measured and the optics and detector unit are in a separate housing on the other side.
As described in the aforesaid U.S. Pat. No. 4,461,576, the measurement of surface level is obtained as follows. The first, or index detector produces an output signal which is amplified and squared to produce a timed index signal during a short portion of each revolution to define an index period. The leading edge of the index signal is used to activate a reference signal in the form of a ramp voltage which increases at a constant rate within each index period. A measurement signal is also generated by the second detector in response to reception of the beam from the laser source after it has been reflected from the surface of the material to be measured. The beam signal occurs once in each rotation of the rotating mirror when the mirror is positioned so that the beam is reflected from the surface and from the parabolic mirror onto the second photodetector. This occurs during the index period at a time which depends upon the level of the material. In accordance with the '576 patent, the ramp reference signal is integrated from its beginning to the end of each cycle and is sampled by the occurrence of the beam signal. This sampled signal provides a value which can be used to determined the level of the material.
It has been found, however, that the analog ramp technique described in the '576 patent has certain inaccuracies, since the signal produced by the second, or beam detector is generally triangular, and can vary in amplitude due to changes in beam intensity or atmospheric conditions in the region of the material being detected.